Atherosclerosis, a chronic inflammatory disease of large and medium sized arteries, is the leading cause of morbidity and mortality in Western countries. Introduction of statins has resulted in a one-third reduction in mortality. Two-thirds of the mortality due to this disease, however, continues despite statin treatment.
Oxidation of low-density lipoproteins (LDL) is a major factor in human atherosclerosis (Witztum and Steinberg (2001) Trends Cardiovasc. Med., 11: 93-102; Witztum and Steinberg (1991) J. Clin. Invest., 88: 1785-1792). Entrapment and oxidation of LDL in the sub-endothelial space, and the subsequent interactions between endothelial cells and monocytes, is a key process in the initiation of atherosclerotic lesion development (Navab et al. (1996) Arterioscler. Thromb. Vasc. Biol., 16: 831-842; Berliner et al. (1995) Circulation 91: 2488-2496). Minimally modified/oxidized-LDL (MM-LDL) contains biologically active molecules that are capable of inducing endothelial cells to produce inflammatory agents, such as chemokines, adhesion molecules, and growth factors. These inflammatory molecules promote the recruitment and adhesion of monocytes to the endothelial cells Berliner et al. (1995) Circulation 91: 2488-2496). Several biologically active oxidized phospholipids have been identified in MM-LDL and in atherosclerotic lesions of animal models (Watson et al. (1995) J. Clin. Invest., 95: 774-782; Watson et al. (1995) J. Clin. Invest., 96: 2882-2891; Watson et al. (1997) J. Biol. Chem., 272: 13597-13607; Watson et al. (1999) J. Biol. Chem., 274: 24787-24798; Leitinger et al. (1999) Arterioscler. Thromb. Vasc. Biol., 19: 1291-1298; Subbanagounder et al. (2000) Free Radic. Biol. Med., 28: 1751-1761). Oxidized-L-α-1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine (ox-PAPC) and three of its components, 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC), 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine (PGPC) and 1-palmitoyl-2-(5,6-epoxyisoprostane E2)-sn-glycero-3-phosphocholine (PEIPC) (Watson et al. (1999) J. Biol. Chem., 274: 24787-24798; Leitinger et al. (1999) Proc. Natl. Acad. Sci. U.S.A. (1999) 96:12010-12015; Subbanagounder et al. (2000) Arterioscler. Thromb. Vasc. Biol., 20: 2248-2254), induce monocyte binding to endothelial cells and play a major role in the activation of endothelial cells by MM-LDL. Subsequent to the discovery of these molecules, a series of other oxidized phospholipids formed by the oxidation of an unsaturated fatty acid at the sn-2 position of the phospholipid have been identified with similar biologic activities (Berliner and Watson (2005) N. Engl. J. Med., 353: 9-11).
The inverse relationship between HDL and the risk of atherosclerosis is well established. Although there does not appear to be a single explanation for the anti-atherogenic role of HDL, it has become clear that the functional status of HDL, which is largely dependent on its protein components, is an important determinant of coronary heart disease (CHD) (Navab et al. (2001) Arterioscler. Thromb. Vasc. Biol., 21: 481-488). Paraoxonase 1 (PON1), lecithin-cholesterol acyltransferase (LCAT), platelet-activating factor acetyl hydrolase (PAF-AH), proteinase (elastase-like), phospholipase D, albumin, apoJ and apoA-I are proteins in HDL with anti-atherogenic properties capable of preventing MM-LDL formation. HDL has been shown to have a role in preventing LDL oxidation. HDL was shown to inhibit the mild oxidation of LDL and consequently inhibit the production of the potent monocyte chemoattractant MCP-1 by human artery wall cells.
HDL can exist as an anti-inflammatory molecule or a pro-inflammatory molecule depending on the context and environment. The acute phase reaction (APR) in rabbits and humans can convert HDL from an anti-inflammatory to a pro-inflammatory form, i.e. HDL looses its ability to protect against LDL-induced inflammation and in its pro-inflammatory state HDL actually promotes LDL-induced inflammation. Without being bound to a particular theory, it is believed that, under basal conditions, HDL serves an anti-inflammatory role but during APR there is loss of anti-oxidant enzyme activities, damage to apoA-I, and displacement and/or exchange of proteins associated with HDL resulting in a pro-inflammatory HDL. It has been shown, for example, that an atherogenic diet in mice that are genetically susceptible to atherosclerosis (but not in mice genetically resistant to atherosclerosis) converts HDL from anti-inflammatory to pro-inflammatory (Navab et al. (1997) J. Clin. Invest., 99: 2005-2019). Several studies have since shown the presence and nature of pro-inflammatory HDL in animal models (Castellani et al. (1997) J. Clin. Invest., 100: 464-474, reviewed in Navab et al. (2005) Ann. Med., 37: 173-178).
HDL from C57BL/6J mice (a strain that is genetically susceptible to dietary-induced atherosclerosis) was anti-inflammatory in mice on a chow diet but was pro-inflammatory when the mice were on an atherogenic diet (Shih et al. (1996) J. Clin. Invest., 97: 1630-1639). In contrast, the HDL from atherosclerosis-resistant C3H/HeJ (C3H) mice was anti-inflammatory whether the mice were on a chow or atherogenic diet (Id.). All of the mouse models studied to date that develop atherosclerosis with macrophage-rich lesions have pro-inflammatory HDL. These include C57BL/6J mice on an atherogenic diet (Id.), transgenic mice that overexpress apoA-II on a chow diet (Castellani et al. (1997) J. Clin. Invest., 100: 464-474), PON1 null mice fed an atherogenic diet (Id.), apoE null mice (Navab et al. (1997) J. Clin. Invest., 99: 2005-2019), combined apoE null and PON1 null mice on a chow diet (Castellani et al. (1997) J. Clin. Invest., 100: 464-474), LDL receptor (LDLR) null mice fed a high-fat diet (Navab et al. (1997) J. Clin. Invest., 99: 2005-2019), and transgenic mice that overexpress sPLA2 (Leitinger et al. (1999) Arterioscler. Thromb. Vasc. Biol., 19: 1291-1298). In genetically distinct mouse models, all of which were hyperlipidemic, but none of which developed atherosclerosis HDL was found to be anti-inflammatory (Navab et al. (2005) Ann. Med., 37: 173-178). On the other hand, seven other mouse models all of which developed atherosclerosis characterized by macrophage-rich lesions had pro-inflammatory HDL (Id.), suggesting that the anti- or pro-inflammatory nature of HDL function is a more sensitive indicator of the presence or absence of atherosclerosis than HDL cholesterol levels. The quality and function of HDL has become an attractive target for emerging new therapies (Castellani et al. (1997) J. Clin. Invest., 100: 464-474; Navab et al. (2005) Ann. Med., 37: 173-178; Ridker (2002) Circulation, 105: 2-4; Ansell et al. (2003) Circulation, 108: 2751-2756).
Although a number of proteins and enzyme activities have been associated with HDL, little is known of what particular protein profiles are associated with pro-inflammatory HDL.